One problem in semiconductor lasers is that the threshold current Ith required for them to reach their lasing threshold increases strongly with increasing temperature. Also, the rate of increase of the optical power of light output by a semiconductor laser above its lasing threshold with increasing input current, called slope efficiency, decreases with increasing temperature. Thus, if the current supplied to a semiconductor laser is kept constant, the power of light output by the laser drops considerably as its temperature increases. These effects are caused by intrinsic processes such as the thermal broadening of the gain spectrum, non-radiative Auger recombination and Inter-valence Band Absorption. Removal of these processes has defied more than thirty years of expensive research and development. As a result, in many applications, the temperature of a semiconductor laser has to be carefully controlled using large, expensive and power-hungry cooling units, and/or the current supplied to a semiconductor laser has to be carefully controlled to maintain a light output of constant optical power. The cooling units and control circuitry often use an order of magnitude more electrical power than the laser itself.
FIG. 1 shows how the optical power of light output by a conventional 1.5 μm InGaAsP semiconductor laser varies with the input current supplied to the laser at a plurality of different temperatures. The threshold current Ith at each temperature is the elbow of the curve, at which the emission of coherent light by the laser begins. Slope efficiency at each temperature represents the change in power of the light output in relation to the change in input current supplied to the laser. Both the threshold current Ith and the slope efficiency are sensitive to temperature, and usually particularly so at and around room temperature. It can be seen that the threshold current Ith increases with increasing temperature. It can also be seen that for a constant bias current the optical power of the light output decreases with increasing temperature due to both increasing threshold current Ith and decreasing slope efficiency. The relationship between optical power of the light output and temperature at a fixed bias current is shown in FIG. 2.